Hello, I have a pair of 8417 amps , I have done some of the upgrades posted on triodeel alowing me to use 6550, el 34 and the orignal 8417's . The problem I have is that one amp has a hum at the speaker when powered on. I disaconnected it from the pre-amp and the hum is still there. Can anybody give deas as to where to look for the problem. I am a mechanic , not to knoweldgeable with electronisc, but have a tech friend that will do the job. Just want to give ideas to check.

thx Malcolm

Positron

10th September 2003 05:53 PM

Some thoughts

Dear Malcolm,
Got some thoughts. I assume they are both wired exactly the same way. Leads are strung the same etc.

1) What frequency is the hum?

2) If 60hz, ICs from one channel nearer to power sources?

3) Substitute the tubes in the bad channel. Could have heater to cathode leakage.

4) If 120hz hum, check power supply caps?

5) Poor solder connection? Check for good grounds.

6) Hum loop problems, but just one chassis??

SY

10th September 2003 06:03 PM

Did you disturb any of the grounds or power supply when doing the mods?

I'd be checking waveforms on power rails, too.

Look at side-to-side balance in the push-pull stages.

dreaded 42

10th September 2003 10:32 PM

Re: Some thoughts

Quote:

Originally posted by Positron Dear Malcolm,
Got some thoughts. I assume they are both wired exactly the same way. Leads are strung the same etc.

1) What frequency is the hum?

2) If 60hz, ICs from one channel nearer to power sources?

3) Substitute the tubes in the bad channel. Could have heater to cathode leakage.

4) If 120hz hum, check power supply caps?

5) Poor solder connection? Check for good grounds.

6) Hum loop problems, but just one chassis??

I don't know what frequency it is , and it is only the one amp, have brand new 6550' s in it, does it even when i swap it with el 34's . Thanks for your replies.

Malcolm

Original Burnedfingers

11th September 2003 03:45 AM

Malcolm

Were both amps quiet after the modifications? If not then take the bottoms off of both amps and compare the wiring to see if there are any mistakes or cold solder joints. If this is a problem that just started then it possibly could be tube related or power supply related.

Just out of currosity is there a jumper from pin 1 to pin 8 on your output tube sockets? If I am correct there should be one in order to run EL34 tubes in this amp.

Are both sections of the 12AX7 and 12BH7 being used? The factory wiring only used one side of each of these tubes. The modification suggests the usage of both sides but it isn't essential to do so.

Beg or borrow someones meter and check the power supply voltages between the two amps and this may give you an idea as to the condition of the power supply caps in the unit making the noise. If the voltage is down it probably suggests that a replacement of the supply caps is in order.

This is all the ideas I can give you right now until you provide some results of your testing.